I would suggest painting the engine compartment, fuel tank compartment, and floor of the radio compartment with epoxy thinned with Isopropyl alcohol. You can find the 99% stuff at the drugstore. Rubbing alcohol will work also but not as well.

I would try to reinforce the firewall around the corners. If you cannot get in there to put triangle stock around the edges, Put regular 30 min epoxy down in the tank compartment with the nose down. Roll the plane around so the epoxy flows into all the corners.

Enjoy,

Jim